This invention relates to a method and apparatus for gamma correction and independent modification of black and white (pedestal and gain, respectively) levels.
Typically, gamma correction is accomplished by implementing a piecewise (step by-step) linear transfer function utilizing a load resistor network for a transistor. The network is interconnected with diodes to provide a plurality of gain break points at particular pre-determined voltage values. A gain/voltage characteristic is generated by the gamma corrector and is selected to compensate for the non linearities of the transducer (e.g. camera or monitor). While this yields an acceptable gamma curve, it does not operate effectively when used in systems requiring the matching of several channels, for example, in a color television channel having red, green and blue channels. The result of gamma correction mismatch is a color shift for various signal amplitudes. This is most noticeable on monochrome signals which appear in color if the gain of all three channels is not the same.
Additional problems occur when it is desired to have a variable gamma value. Gamma values can be made variable using a plurality of resistors of different values, but the resolution of such systems is limited by the number of resistors available in the circuit. In addition, while circuits do exist that digitally select the resistor values to allow control of the gamma value, the gamma correction circuit is still an analog circuit and therefore includes the inherent stability problems of such analog circuits. Analog gamma correctors are made with transistor amplifiers and use resistor-diode combinations to set the various gain break points. Temperature and component age affect these components such that their characteristics do not remain constant.
Digital gamma correctors are also known in the art. It is known, for example, to input data relating to a desired transfer characteristic into a random access memory (RAM) and then, using the data stored in the RAM, to make adjustments to the video signal. See, for example, U.S. Pat. No. 4,394,688 to Iida et al. (hereinafter, "Iida") and U.S. Pat. No. 4,786,968 to Kutner (hereinafter, "Kutner"). U.S. Pat. No. 4,568,978 to Cosh teaches "real time" gamma correction without the use of RAMs. While these prior art systems eliminate the need of the above-mentioned resistor network/diode configuration for gamma correction, they do not allow independent adjustment of black and white levels. In addition, the time required to compute the data to be loaded to the RAM in the Iida Patent and the Kutner Patent slows down the correction process.
The gamma value level is not the only value that is of concern to users of television systems. The black level, or pedestal, and the white level, or gain, are also signals that affect the signal and which are routinely adjusted in the generation of a television signal.